Method and system for preparing tissue samples for histological and pathological examination

ABSTRACT

Viable biological material is cryogenically preserved (cryopreservation) by immersing the material in a tank of cooling fluid, and circulating the cooling fluid past the material at a substantially constant predetermined velocity and temperature to freeze the material. The material may either be directly plunged into the cooling fluid without preparation, or chemically prepared prior to freezing. A method according to the present invention freezes the biologic material quickly enough to avoid the formation of ice crystals within cell structures (vitrification) and allows the samples to maintain anatomical structure and remain biochemically active after thaw. The temperature of the cooling fluid is preferably between −20 degrees centigrade and −30 degrees centigrade, which is warm enough to minimize the formation of stress fractures and other artefacts in cell membranes due to thermal changes. Cells frozen using a method according to the present invention have been shown to have a significantly less cellular and intercellular damage than cells frozen by other cryopreservation methods used for pathological and histological techniques. Because the present invention can freeze biological material such that the material is vitrified, biochemical activity within the cell is not lost after freezing and thus various embodiments of the present method may be employed in a system to prepare biological material for the newer techniques of cryopathology and immunohistochemistry in the areas of research and patient care.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a Continuation of U.S. patent application Ser. No.10/034,999, entitled “Method And System For Preparing Tissue Samples ForHistological And Pathological Examination”, which was filed on Dec. 28,2001. This application claims benefit under 35 U.S.C.§ 119 of thefollowing U.S. provisional patent application Serial No. 60/259,418,entitled “Method And System For Preparing Tissue Samples ForHistological And Pathological Examination”, which was filed on Jan. 2,2001.

FIELD OF THE INVENTION

[0002] The present invention relates generally to cryogenic preservationand more particularly to a method of preserving for examination anddiagnostic purposes.

BACKGROUND OF THE INVENTION

[0003] Biological materials such as tissues are subjected to varioustreatments in an histology laboratory to prepare specimens on slides forviewing under a microscope. Pathologists carefully examine the slidesand report their findings, which aids physicians in the diagnosis ofdisease or disease processes. Histopathology has traditionally reliedupon examination of samples prepared by one of two basic methods. In thefirst histological method, samples undergo significant processing in thelaboratory, such as fixation to preserve tissues, dehydration to removewater from tissues, infiltration with embedding agents such as paraffin,embedment, sectioning or cutting sections of the tissue for placement ona slide, mounting the sections, and staining the sections to enhancedetails. The second method, cryogenic preparation, significantly reducesthe processing of the first method in that it generally involves snapfreezing in a cold liquid or environment, sectioning, mounting, andstaining.

[0004] While the first method yields significantly superiorvisualization, it requires an extended period of time for processing,generally a minimum of 18 to 24 hours. Thus this method cannot beapplied in situations where a rapid diagnosis of a pathologic process isrequired, such as during a surgical procedure. Additionally, theprocessing techniques employed may destroy all or part of the biologicalactivity of the tissues.

[0005] The second method has the advantage of speed (30 minutes to 1hour), however tissue specimens prepared using cryogenic preparation areoften subject to cellular damage due to ice crystal formation, which canalso cause the loss of biological function of molecules of interestwithin the tissues, and overall loss of tissue integrity manifested asdegraded anatomical structure. Many commercial pathology laboratoriesdiscourage the use of frozen tissue for immunohistochemistry in all butspecial circumstances, because ice crystal formation in stored tissuecauses many abnormal artifacts within the sample which make diagnosticinterpretation quite difficult, or even impossible in some cases.

[0006] With the advent of poly- and then monoclonal antibodies, thefocus of both traditional microscopic histology and pathology hasshifted from simple subjective observation, to direct objective stainingprocedures. These newer immunohistochemistry (IHC) techniques help indetermining diagnosis when histopathology alone proves inconclusive.However, IHC techniques are dependent on biologically intact receptorswithin the specimen for proper staining to occur. Therefore it isdesirable to utilize a method of tissue specimen preparation that doesnot limit the amount of active biological material present afterpreparation is complete.

SUMMARY OF THE INVENTION

[0007] Therefore, what is needed is an improved way to cryogenicallypreserve viable single cells, tissues, organs, nucleic acids, or otherbiologically active molecules, that avoids at least some of the problemsinherent in currently available methods. Accordingly, the presentinvention provides a method of cryopreservation for freezing abiochemically active tissue sample by immersing the sample in coolingfluid and circulating the cooling fluid past the material. The coolingfluid is circulated past the tissue sample at a substantially constant,predetermined velocity and temperature to freeze the tissue sample suchthat it is vitrified, yet the tissue sample maintains its anatomicalstructure and remains biochemically active after thaw. In at least oneembodiment, the cooling fluid is maintained at a temperature of betweenabout −20 degrees centigrade and −30 degrees centigrade, and thevelocity of the cooling fluid past the tissue sample is about 35 litersper minute per foot of cooling fluid through an area not greater thanabout 24 inches wide and 48 inches deep. Additionally, at least oneembodiment of the present invention immerses a biologically activetissue sample in cooling fluid to freeze the sample directly to atemperature higher than about −30 degrees centigrade. A furtherembodiment of the present invention provides for circulating the coolingfluid past a multi-path heat exchanging coil submersed in the coolingfluid, where the heat exchanging coil is capable of removing at leastthe same amount of heat from the cooling fluid as the cooling fluidremoves from the tissue sample. At least one embodiment provides asystem for implementing the above mentioned methods.

[0008] An object of at least one embodiment of the present invention isapplication of a method to freeze biological material wherein theformation of ice crystals and stress fractures is avoided, and cellularbiochemical function is maintained after freezing.

[0009] An advantage of at least one embodiment of the present inventionis that cryopreservation recovery rates are significantly increased,because biological material is vitrified during freezing.

[0010] Another advantage of at least one embodiment of the presentinvention is that cryopreservation recovery rates are improved, becausebiological material is vitrified at a high enough temperature to avoidthe formation of stress fractures within cell membranes.

[0011] Another advantage of at least one embodiment of the presentinvention is that cryopreservation recovery rates are such that aconsiderably higher percentage of the biological material maintains itsanatomical structure and remains biochemically active after thaw ascompared to currently available methods.

[0012] An additional advantage of at least one embodiment of the presentinvention is that cryopreservation recovery rates are such that thebiological material samples lend themselves to the application ofsectioning, processing and subsequent histological, ultrastructural, andimmunohistochemistry examination in shorter periods of time thantraditional pathology techniques, thus shortening time to results.

[0013] A further advantage of at least one embodiment of the presentinvention is that once frozen, existing cryopreservation storagefacilities and mechanisms can be used to store the frozen biologicalmaterials.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Other objects, advantages, features and characteristics of thepresent invention, as well as methods, operation and functions ofrelated elements of structure, and the combination of parts andeconomies of manufacture, will become apparent upon consideration of thefollowing description and claims with reference to the accompanyingdrawings, all of which form a part of this specification, wherein likereference numerals designate corresponding parts in the various figures,and wherein:

[0015]FIG. 1 is a side view of a chilling apparatus for practicing amethod according to at least one embodiment of the present invention;

[0016]FIG. 2 is a cross sectional view of the chilling apparatusillustrated in FIG. 1 indicating implementation of cooling systemssuitable for freezing relatively large quantities of biologicalmaterial;

[0017]FIG. 2A is a cross sectional view of the chilling apparatus shownin FIG. 1, configured for use with a spiral conveyor according to oneembodiment of the present invention;

[0018]FIG. 3 is a flow diagram illustrating a system implementedaccording to at least one embodiment of the present invention;

[0019]FIG. 4 is a bar chart showing the results of experimentalcomparisons between various prior art freezing methods and a freezingmethod according to a preferred embodiment of the present invention;

[0020]FIG. 5 illustrates views, as seen through a microscope, of themorphological appearance of noncryoprotected grape tissue followingfreeze-thaw cycles of the method of liquid nitrogen and the freezingmethod according to a preferred embodiment of the present invention;

[0021]FIG. 6 illustrates views, as seen through a microscope, of themorphological appearance of heart tissue after freezing using standardcryopreparative techniques, and after application of the methodaccording to a preferred embodiment of the present invention; and

[0022]FIG. 7 is an electron microscope view illustrating the complexultrastructural features such as cellular mitochondria that may be seenafter application of the method according to a preferred embodiment ofthe present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

[0023] In the following detailed description of the preferredembodiments, reference is made to the accompanying drawings which form apart hereof, and in which is shown by way of illustration specificpreferred embodiments in which the invention may be practiced. Theseembodiments are described in sufficient detail to enable those skilledin the art to practice the invention, and it is to be understood thatother embodiments may be utilized and that logical, mechanical, chemicaland electrical changes may be made without departing from the spirit orscope of the invention. To avoid detail not necessary to enable thoseskilled in the art to practice the invention, the description may omitcertain information known to those skilled in the art. The followingdetailed description is, therefore, not to be taken in a limiting sense,and the scope of the present invention is defined only by the appendedclaims.

[0024] Referring first to FIGS. 1 and 2, a chilling apparatus suitablefor practicing a method according to at least one embodiment of thepresent invention is discussed, and designated generally as cooling unit100. Cooling unit 100 preferably comprises tank 110 containing coolingfluid 140. Submersed in cooling fluid 140 are circulators 134 such asmotors 130 having impellers 132, heat exchanging coil 120, and rack 150,which in one embodiment comprises trays 155 for supporting biologicalmaterial to be frozen. Biological material may include, but is notlimited to, viable single cells, tissues and organs, nucleic acids, andother biologically active molecules. Biological material is not requiredto be species-specific. External to tank 110, and coupled to heatexchanging coil 120, is refrigeration unit 190.

[0025] Tank 110 may be of any dimensions necessary to immerse biologicalmaterial to be frozen in a volume of cooling fluid 140, in whichdimensions are scaled multiples of 12 inches by 24 inches by 48 inches.Other tank sizes may be employed consistent with the teachings set forthherein. For example, in one embodiment (not illustrated), tank 110 issized to hold just enough cooling fluid 140, so containers such asvials, test tubes, beakers, graduated cylinders or the like, can beplaced in tank 110 for rapid freezing of suspensions includingbiological materials and cryoprotectants. In other embodiments, tank 110is large enough to completely immerse entire organs and or organisms forrapid freezing. It will be appreciated that tank 110 can be made largeror smaller as needed to efficiently accommodate various sizes andquantities of biological material to be frozen. The biological materialmay be treated with a cryoprotectant prior to being immersed in tank110.

[0026] Tank 110 holds cooling fluid 140. In one embodiment, the coolingfluid is a food-grade solute. Good examples of food-grade quality fluidsare those based on propylene glycol, sodium chloride solutions, or thelike. In another embodiment, the cooling fluid is itself acryoprotectant such as dimethylsulfoxide (DMSO), ethylene glycol,propylene glycol, polyethylene glycol or the like. Note that in someinstances, the cryoprotectant is itself a food-grade quality fluid. Inother embodiments, other fluids, and preferably solutes, are used ascooling fluids. While various containers may be used to hold thebiological material, some embodiments of the present invention providefor the biological material to be directly immersed in the cooling fluidfor rapid and effective freezing. Such direct immersion may simplify thecryopreservation of some tissues and organs.

[0027] In order to freeze biological material while avoiding theformation of ice crystals, one embodiment of the present inventioncirculates cooling fluid 140 past the biological material to be frozen,at a relatively constant rate of 35 liters per minute for every foot ofcooling fluid contained in an area not more than about 24 inches wide by48 inches deep. The necessary circulation is provided by one or morecirculators 134, such as motors 130. In at least one embodiment of thepresent invention, submersed motors 130 drive impellers 132 to circulatecooling fluid 140 past biological material be to frozen. Othercirculators 134, including various pumps (not illustrated), can beemployed consistent with the objects of the present invention. At leastone embodiment of the present invention increases the area and volumethrough which cooling fluid is circulated by employing at least onecirculator 134, in addition to motors 130. In embodiments using multiplecirculators 134, the area and volume of cooling fluid circulation areincreased in direct proportion to each additional circulator employed.For example, in a preferred embodiment, one additional circulator isused for each foot of cooling fluid that is to be circulated through anarea of not more than about 24 inches wide by 48 inches deep.

[0028] Preferably, motors 130 can be controlled to maintain a constant,predetermined velocity of cooling fluid flow past the biologicalmaterial to be preserved, while at the same time maintaining an evendistribution of cooling fluid temperature within +/−0.5 degreescentigrade at all points within tank 110. The substantially constantpredetermined velocity of cooling fluid circulating past the biologicalmaterial provides a constant, measured removal of heat, which allows forthe vitrification of the biological material during freezing. In oneembodiment, cooling fluid properties such as viscosity, temperature, etcetera, are measured and processed, and control signals are sent tomotors 130 to increase or decrease the rotational speed or torque ofimpellers 132 as needed. In other embodiments, motors 130 areconstructed to maintain a given rotational velocity over a range offluid conditions. In such a case, the torque or rotational speed ofimpellers 132 imparted by motors 130 are not externally controlled. Ofnote is the fact that no external pumps, shafts, or pulleys are neededto implement a preferred embodiment of the present invention. Motors130, or other circulators 134, are immersed directly in cooling fluid140. As a result, cooling fluid 140 not only freezes biological materialplaced in tank 110, but cooling fluid 140 also provides cooling formotors 130.

[0029] Heat exchanging coil 120 is preferably a “multi-path coil,” whichallows refrigerant to travel through multiple paths (i.e., three or morepaths), in contrast to conventional refrigeration coils in whichrefrigerant is generally restricted to one or two continuous paths. Inaddition, the coil size is in direct relationship to the cross sectionalarea containing the measured amount of the cooling fluid 140. Forexample, in a preferred embodiment, tank 110 is one foot long, two feetdeep, and four feet wide, and uses a heat exchanging coil 120 that isone foot by two feet. If the length of tank 110 is increased to twentyfeet, then the length of heat exchanging coil 120 is also increased totwenty feet. As a result, heat exchanging coil 120 can be madeapproximately fifty percent of the size of a conventional coil requiredto handle the same heat load. Circulators 134 such as motors 130,circulate chilled cooling fluid 140 over biological material to befrozen, and then transport warmer cooling fluid to heat exchanging coil120, which is submersed in cooling fluid 140. In at least oneembodiment, heat exchanging coil 120 is connected to refrigeration unit190, which removes the heat from heat exchanging coil 120 and thesystem.

[0030] In a preferred embodiment, refrigeration unit 190 is designed tomatch the load requirement of heat exchanging coil 120, so that heat isremoved from the system in a balanced and efficient manner, resulting inthe controlled, rapid freezing of a material. The efficiency of therefrigeration unit 190 is directly related to the method employed forcontrolling suction pressures by the efficient feeding or the heatexchange coil 120 and the efficient output of compressors used inrefrigeration unit 190. This methodology requires very close tolerancesto be maintained between the refrigerant and cooling fluid 140temperatures, and between the condensing temperature and the ambienttemperature. These temperature criteria, together with the design of theheat exchange coil 120, allow heat exchange coil 120 to be fed moreefficiently, which in turn allows the compressor to be fed in a balancedand tightly controlled manner to achieve in excess of twenty fivepercent greater performance from the compressors than that which isaccepted as the compressor manufacturer's standard rating.

[0031] Note that in the embodiment illustrated in FIG. 1, refrigerationunit 190 is an external, remotely located refrigeration system. However,in another embodiment (not illustrated), refrigeration unit 190 isincorporated into another section of tank 110. It will be appreciatedthat various configurations for refrigeration unit 190 may be more orless appropriate for certain configurations of cooling unit 100. Forexample, if tank 110 is extremely large, a separate refrigeration unit190 may be desirable, while a portable embodiment may benefit from anintegrated refrigeration unit 190. Such an integration is only madepossible by the efficiencies achieved by implementing the principles asset forth herein, and particularly the use of a reduced-size heatexchanging coil.

[0032] By virtue of refrigeration unit 190 and heat exchanging coil 120,in a preferred embodiment, the cooling fluid is cooled to a temperatureof between −20 degrees centigrade and −30 degrees centigrade, with atemperature differential throughout the cooling fluid of less than about+/−0.5 degrees centigrade. In other embodiments, the cooling fluid iscooled to temperatures outside the −20 degrees centigrade to −30 degreescentigrade range in order to control the rate at which a substance is tobe frozen. Other embodiments control the circulation rate of the coolingfluid to achieve desired freezing rates. Alternatively, the volume ofcooling fluid may be changed in order to facilitate a particularfreezing rate. It will be appreciated that various combinations ofcooling fluid circulation rate, cooling fluid volume, and cooling fluidtemperature can be used to achieve desired freezing rates.

[0033] Referring now to FIG. 2, a cross sectional view of the chillingapparatus illustrated in FIG. 1 indicating implementation of coolingsystems suitable for freezing relatively large quantities of biologicalmaterial; an embodiment of cooling system 100 suitable for freezingrelatively large quantities of biological material is discussed.Reference numerals in FIG. 2 that are like, similar, or identical toreference numerals in FIG. 1 indicate like, similar, or identicalfeatures. Tank 110 contains cooling fluid 140, into which rack 150 maybe lowered. Rack 150 is movably coupled to rack support 210, such thatrack 150 may be raised or lowered to facilitate the placement ofsubstances into tank 110.

[0034] In use, biological material to be frozen is placed in trays 155of rack 150. Preferably, trays 155 are constructed of wire, mesh, orotherwise, so that cooling fluid 140 may freely circulate over, under,and/or around items placed thereon. Preferably, once the cooling fluidis chilled to a desired temperature, rack support 210 lowers rack 150into tank 110, in order to submerge trays 155 in cooling fluid 140.Lowering rack 150 may be accomplished manually or using various gear,chain, and/or pulley configurations known to those skilled in the art.Circulators 134 circulate cooling fluid 140 across substances placed intrays 155 to provide quick and controlled freezing. It will beappreciated that other arrangements for immersing biological materialinto tank 110 may be employed, and that use of an automatic loweringsystem is not necessarily preferred for use in all circumstances.

[0035] Referring now to FIG. 2A, an embodiment of the present inventionemploying a multi-tiered spiral path conveyor system is discussed. Asillustrated, spiral conveyor 200 may be configured to fit inside tank110 in order to submerge biological material into cooling fluid 140. Inuse, once the cooling fluid is chilled to a desired temperature,materials to be frozen are fed into an input feed 160 where they aretaken onto conveyor belt 170. The material travels from input feed 160,into the cooling fluid 140 on downward spiral 175, out of cooling fluid140 on upward spiral 176, and out of spiral conveyor at output feed 180.As noted earlier, the cooling fluid 140 is preferably kept at a constantpredetermined temperature, and circulated at a rate that ensures rapid,safe freezing of material to be frozen. The time the material spendssubmerged in cooling fluid 140 can be varied by adjusting the driveunit, 230, or by other suitable means. Ideally, the speed of conveyorbelt 170, in combination with the temperature and circulation rate ofcooling fluid 140, will be adjusted so that exactly the desired amountof heat will be removed from materials as they travel through tank 110on the multi-tiered spiral path conveyor system 200.

[0036] Referring now to FIG. 3, a method according to one embodiment ofthe present invention is illustrated, and designated generally byreference numeral 300. The illustrated method begins at step 310, wherecooling fluid is circulated past a heat exchange coil. The heat exchangecoil is operably coupled to a refrigeration system as discussed above,and is used to reduce the temperature of the cooling fluid as thecooling fluid is circulated past the heat exchange coil. In step 320,the temperature of the cooling fluid is measured, and the methodproceeds to step 330, where it is determined whether the temperature ofthe cooling fluid is within an optimal temperature range. This optimalcooling fluid temperature range may be different for differentapplications, however a preferred optimal temperature range for manyapplications is between −20 degrees centigrade and −30 degreescentigrade.

[0037] Should the cooling fluid temperature be determined not to bewithin an optimal, predetermined temperature range, step 335 isperformed. In step 335, the heat exchanging coil is cooled by arefrigeration unit, and the method returns to step 310, in which thecooling fluid is circulated past the heat exchange coil in order tolower the temperature of the cooling fluid. Preferably, steps310,320,330, and 335 are performed continually until the cooling fluidreaches the optimal temperature range.

[0038] The temperature of the cooling fluid used to freeze thebiological material is an important element of at least one embodimentof the present invention. In order to achieve vitrification usingconventional processes, biological material is generally quenched inliquid nitrogen, at a temperature of −196 degrees centigrade. Such adrastic change in temperature over a very short period of time freezeswater within cell structures so quickly that ice crystals do not have achance to form. However, freezing biological material by quenching inliquid nitrogen can cause stress fractures in cellular membranes,thereby limiting the usefulness of quenching in liquid nitrogen forcryopreservation. Since the temperatures used in a preferred embodimentof the present invention are between −20 degrees centigrade and −30degrees centigrade, stress fractures due to temperature changes areminimized, and vitrification can be achieved with far less damage tocellular membranes.

[0039] While the cooling fluid is being cooled to the propertemperature, biological material to be frozen may be chemically preparedfor freezing in step 305. It will be appreciated that materials to beused for pathology do not normally require chemical preparation, andforegoing step 305 by plunging materials to be frozen directly into acooling fluid is consistent with the teachings set forth herein. Asnoted earlier, biological material includes, but is not limited to,viable single cells, tissues and organs, nucleic acids, and otherbiologically active molecules. The biological material does not have tobe species specific. Chemically preparing the biological material mayinclude pretreatment of the biological material with agents(stabilizers) that increase cellular viability by removing harmfulsubstances secreted by the cells during growth or cell death. Usefulstabilizers include those chemicals and chemical compounds, many ofwhich are known to those skilled in the art, which sequester highlyreactive and damaging molecules such as oxygen radicals.

[0040] Chemically preparing biological material may also include anacclimation step (not illustrated). During or at some time afterpretreatment, the biological material to be preserved may be acclimatedto a temperature which is reduced from culturing temperatures, but stillabove freezing. This may help prepare the biological material for thecryopreservation process by retarding cellular metabolism and reducingthe shock of rapid temperature transition. Note well, however, than anacclimation step is not required in order to practice the presentinvention.

[0041] In a preferred embodiment, chemically preparing biologicalmaterial for freezing includes loading the biological material with acryoprotectant. Loading generally involves the equilibration ofbiological material in a solution of one or more cryoprotectants.Substances utilized during loading may be referred to as loading agents.Useful loading agents may include one or more dehydrating agents,permeating and non-permeating agents, and osmotic agents. Bothpermeating agents such as DMSO and ethylene glycol, and a combination ofpermeating and non-permeating osmotic agents such as fructose, sucroseor glucose, and sorbitol, mannitol, or glycerol can be used. It will beappreciated that other suitable cryoprotectants may be employedconsistent with the objects of the present invention.

[0042] After the cooling fluid reaches a proper temperature, step 315 isperformed, in which the chemically prepared biological material isimmersed in cooling fluid. As noted earlier, the biological material maybe held in a container, or placed directly into the cooling fluid. Themethod then proceeds to step 337, in which a circulator, such as asubmersed motor/impeller assembly or pump, is used to circulate thecooling fluid at the velocity previously discussed, past the immersedbiological material. As the cooling fluid passes by the biologicalmaterial, heat is removed from the material, which is at a highertemperature than the temperature of the cooling fluid, and istransferred to be cooling fluid, which transports the heat away from thebiological material to be frozen. According to at least one embodimentof the present invention, a substantially constant circulation ofcooling fluid past the biological material to be frozen should bemaintained in order to freeze the prepared biological material such thatthe prepared material is vitrified.

[0043] After the cooling fluid is circulated past the biologicalmaterial to be frozen, step 339 is performed. Step 339 adjusts thevelocity of the cooling fluid as necessary to account for changes in thecooling fluid viscosity, temperature, and the like. Preferably, thevelocity of the cooling fluid is held constant by adjusting the forceprovided by one or more circulators. Once the biological material hasreached the desired frozen state, it is removed as shown in step 340.After the material is removed from the cooling fluid in step 340 bymeans previously discussed, it may be sectioned and thawed forhistological, ultrastructural, and immunohistochemistry examinations,such as fluorescent labeled antibody staining.

[0044] The steps illustrated in FIG. 3 are shown and discussed in asequential order. However, the illustrated method is of a nature whereinsome or all of the steps are continuously performed, and may beperformed in a different order. For example, at least one embodiment ofthe present invention uses a single circulating motor to circulate thecooling fluid. In such an embodiment, cooling fluid is circulated past aheat exchanging coil as in step 310, and past the biological material tobe preserved in step 337 at the same time. In addition, one embodimentof the present invention measures cooling fluid temperatures,viscosities, and other fluid properties continually, and at multiplelocations within the system.

[0045] In yet another embodiment, some properties of the cooling fluidare not directly measured. Rather, the change in cooling fluidproperties is determined indirectly from the rotational speed of acirculation motor. If the motor is turning at a slower rate, thenadditional power can be supplied to the motor to return the motor to thedesired rotational speed, thereby compensating for the change in coolingfluid properties. In at least one embodiment, a motor is configured tomaintain a substantially constant rate of rotation. This substantiallyconstant rate of rotation will result in a substantially constant rateof cooling fluid circulation.

[0046] A test of one embodiment of the present invention was performedin which five milliliters (5 ml) of water was frozen in a graduatedcontainer. Upon freezing, there was less than one percent increase intotal volume, much less than expected with conventional freezing. Inanother test, ice was frozen in sheets in a conventional freezer, and ina cooling system according to a preferred method of the presentinvention. After freezing, the ice was examined under dark microscope.As expected, the conventional ice displayed a crystalline pattern,whereas the ice frozen according to the principles of the presentinvention exhibited no light displacement, indicating little to no icecrystal formation.

[0047] Refer now to FIG. 4, in which experimental results comparingvarious cryopreservation methods are compared. Bar graph 400 comparesthe number of individual cells damaged by use of four differentcryopreservation methods B, C, D, and E against a control group A. Nocryopreservation was performed on control group A, method B used aconventional freezer to freeze cells to a temperature of −20 degreescentigrade, method C used an ultralow freezer to freeze cells to atemperature of −80 degrees centigrade, method D used liquid nitrogen tofreeze cells to a temperature of −196 degrees centigrade, and method Eused a preferred embodiment of the present invention to freeze cells toa temperature of −25 degrees centigrade.

[0048] The results of the experiments, shown in bar graph 400, usedplant tissue (seedless grapes) which were frozen by the conventionalmethods previously discussed, as well as by the method as embodied bythe present invention, without any form of preparation orcryoprotectant. The frozen plant tissue was then thawed and thinsections were cut and examined, unstained, using phase-contrastmicroscopy. Plant tissue was employed in the experiments because grossdistortion of the tissue by ice crystal formation or water expansioncaused by freezing would disrupt the tissue's cell wall structure andcould be readily observed. The results, as illustrated in FIG. 4,clearly show the superiority of the method performed according to apreferred embodiment of the present invention. As expected, the control,A, exhibited no cellular damage. Method B, the −20 C freezer, exhibiteddamage in approximately 45% of the cellular wall structures; method C,the −80 C freezer, exhibited damage in approximately 55% of the cellularwall structures; method D, liquid nitrogen, exhibited damage inapproximately 59% of the cellular wall structures. However, the methodperformed according to a preferred embodiment of the present inventionexhibited only about 12.5% cellular damage.

[0049] The superiority of the method performed according to a preferredembodiment is also seen in FIG. 5, which illustrates views, as seenthrough a phase-contrast microscope, of the morphological appearance ofnoncryoprotected grape tissue following freeze-thaw cycles of the methodof liquid nitrogen and the freezing method according to a preferredembodiment of the present invention. Note in FIG. 5 the altered form andstructure of the tissue indicating cellular wall damage is seen to beconsiderably less in the freeze-thaw method performed according to apreferred embodiment than that seen in the view of tissue freeze-thawcycled with a method using liquid nitrogen.

[0050] Referring now to FIG. 6, views, as seen through a microscope, ofthe morphological appearance of heart tissue after freezing usingstandard cryopreparative techniques, and after application of the methodaccording to a preferred embodiment of the present invention isdiscussed. FIG. 6 illustrates the results of a different experimentwhich was performed on tissue samples collected post-mortem from miceand canine cadavers. Tissue samples were collected from five organsystems: ovarian, heart, liver, kidney, and lung. Tissues were preparedfor conventional histology, cryosectioning, or ultrastructuralexamination using standard freezing techniques, and also followingfreezing by the method performed according to a preferred embodiment ofthe present invention. The resulting sections were then evaluated by atrained clinical pathologist. As expected, samples that were neverfrozen exhibited superior morphology upon histological evaluation.However, the pathologist report states that tissue frozen according tothe method of a preferred embodiment of the present invention was atleast as well preserved as tissue using standard cryogenic technology,and further that several types of tissue, most notably kidney and muscle(heart) demonstrated marked improvement in tissue integrity when frozenaccording to the method embodied by the present invention. FIG. 6clearly indicates that the standard cryopreparative technique hasnumerous artifacts, such as “accordion folds” 605 seen within the heartmuscle sample, as compared to the heart muscle sample which underwentthe method as embodied by the present invention.

[0051] Refer now to FIG. 7, in which an electron microscope viewillustrates the complex ultrastructural features, such as cellularmitochondria 705, seen after application of the method according to apreferred embodiment of the present invention as compared to a controlwhich was never frozen. The electron microscope views illustrated inFIG. 7 clearly show little if any difference between the tissues frozenby the method according to a preferred embodiment of the presentinvention and control tissue which had never been frozen. Additionally,tissues frozen by the standard techniques of liquid nitrogen ormechanical freezing (not illustrated) exhibited significantly moredamage upon examination than those of tissues frozen by the methodaccording to a preferred embodiment of the present invention.

[0052] As stated earlier, a major problem with frozen sections createdusing the current technology is the loss of specific chemical reactionsupon freezing. Loss of this activity renders these samples essentiallyuseless for the more modern techniques of immunohistochemistry basedupon antibody stain. An experiment which was conducted using afluorescent labeled antibody (5.1H11, a human NCAM that is musclespecific) demonstrated that primary porcine satellite cells which werepreviously stained for fluorescence with this antibody continued tofluoresce after freezing when prepared according to the method of apreferred embodiment of the present invention. However, cells frozen inliquid nitrogen failed to fluoresce after thaw. The results of thisexperiment indicate that the method of a preferred embodiment will allowthe newer techniques of cryopathology and immunohistochemistry to beapplied in the areas of research and patient care.

[0053] Because the present invention can freeze biological material suchthat the material is vitrified, the formation of stress fractures incellular membranes is minimized, and chemical activity within the cellis not lost after freezing, various embodiments of the present inventionmay find application in other medical fields with proper chemicalpreparation, such as skin grafts, cornea storage, circulatory vesselstorage, freezing of transplant tissues, and infertility treatment, aswell as in the investigation of molecular regeneration disease (cancer).

[0054] Although an embodiment of the present invention has been shownand described in detail herein, along with certain variants thereof,many other varied embodiments that incorporate the teachings of theinvention may be easily constructed by those skilled in the art.Accordingly, the present invention is not intended to be limited to thespecific form set forth herein, but on the contrary, it is intended tocover such alternatives, modifications, and equivalents, as can bereasonably included within the spirit and scope of the invention.

What is claimed is:
 1. A method comprising: freezing a biochemicallyactive tissue sample, wherein freezing includes: immersing the tissuesample in cooling fluid; circulating the cooling fluid past the tissuesample at a substantially constant predetermined velocity andtemperature to freeze the tissue sample such that the tissue sample isvitrified; and wherein the tissue sample maintains its anatomicalstructure and remains biochemically active after thaw; thawing thetissue sample; and examining the thawed tissue sample.
 2. The method asin claim 1, further comprising sectioning the tissue sample.
 3. Themethod as in claim 1, wherein examining the thawed tissue sampleincludes histological examination.
 4. The method as in claim 1, whereinexamining the thawed tissue sample includes ultrastructural examination.5. The method as in claim 1, wherein examining includes the use ofimmunohistochemistry examination.
 6. The method as in claim 5, whereinimmunohistochemistry includes fluorescent labeled antibody staining. 7.The method as in claim 1, wherein more than about 55 percent of thetissue sample exhibits no damage to cellular anatomical structure andremains biochemically active after thaw.
 8. The method as in claim 1,wherein more than about 45 percent of the tissue sample exhibits nodamage to cellular anatomical structure and remains biochemically activeafter thaw.
 9. The method as in claim 1, wherein more than about 85percent of the tissue sample maintains its anatomical structure andremains undamaged after thaw.
 10. The method as in claim 1, wherein thecooling fluid is maintained at a temperature of between about −20degrees centigrade and about −30 degrees centigrade.
 11. The method asin claim 1, wherein the velocity of the cooling fluid past the tissuesample is about 35 liters per minute per foot of cooling fluid throughan area not greater than about 24 inches wide and 48 inches deep. 12.The method as in claim 1, wherein, the cooling fluid is circulated by amotor/impeller assembly immersed in the cooling fluid.
 13. The method asin claim 1, further comprising circulating the cooling fluid past amulti-path heat exchanging coil submersed in the cooling fluid, andwherein the heat exchanging coil is capable of removing at least thesame amount of heat from the cooling fluid, as the cooling fluid removesfrom the tissue sample.
 14. A method for use in preparing a tissuesample for examination, the method comprising: immersing a biologicallyactive tissue sample in cooling fluid; and freezing the tissue sampledirectly to a temperature higher than about −30 degrees centigrade bycirculating the cooling fluid past the tissue sample at a substantiallyconstant predetermined velocity and temperature such that the tissuesample is vitrified, the tissue sample maintains its anatomicalstructure, and the tissue sample remains biochemically active afterthaw.
 15. The method as in claim 14, further comprising sectioning thetissue sample.
 16. The method as in claim 14, further comprising thawingthe tissue sample.
 17. The method as in claim 14, wherein examinationincludes histological examination.
 18. The method as in claim 14,wherein examination includes ultrastructural examination.
 19. The methodas in claim 14, wherein examination includes the use ofimmunohistochemistry examination.
 20. The method as in claim 19, whereinimmunohistochemistry includes fluorescent labeled antibody staining. 21.The method as in claim 14, wherein more than about 40 percent of thetissue sample maintains its anatomical structure and remainsbiochemically active after thaw.
 22. The method as in claim 14, whereinmore than about 80 percent of the tissue sample maintains its anatomicalstructure and remains biochemically active after thaw.
 23. The method asin claim 14, wherein more than about 85 percent of the tissue samplemaintains its anatomical structure and remains undamaged after thaw. 24.The method as in claim 14, wherein the cooling fluid is maintained at atemperature of between about −20 degrees centigrade and about −30degrees centigrade.
 25. The method as in claim 14, wherein the velocityof the cooling fluid past the tissue sample is about 35 liters perminute per foot of cooling fluid through an area not greater than about24 inches wide and 48 inches deep.
 26. The method as in claim 14,wherein, the cooling fluid is circulated by a motor/impeller assemblyimmersed in the cooling fluid.
 27. The method as in claim 14, furthercomprising circulating the cooling fluid past a multi-path heatexchanging coil submersed in the cooling fluid, and wherein the heatexchanging coil is capable of removing at least the same amount of heatfrom the cooling fluid, as the cooling fluid removes from the tissuesample.
 28. A system for use in preparing a tissue sample forexamination, the system comprising: a cooling fluid reservoir configuredto receive a biochemically active tissue sample for immersion in coolingfluid; one or more cooling fluid circulators configured to circulatesaid cooling fluid; a heat exchanging coil for removing heat from saidcooling fluid; a refrigeration unit configured to remove heat from saidheat exchanging coil; and wherein said cooling fluid reservoir, said oneor more circulators, and said refrigeration unit cooperate to freeze thetissue sample directly to a temperature higher than about −30 degreescentigrade by circulating the cooling fluid past the tissue sample at asubstantially constant predetermined velocity and temperature such thatthe tissue sample is vitrified, the tissue sample maintains itsanatomical structure, and the tissue sample remains biochemically activeafter thaw.
 29. The system as in claim 28, wherein examination includeshistological examination.
 30. The system as in claim 28, whereinexamination includes ultrastructural examination.
 31. The system as inclaim 28, wherein examination includes the use of immunohistochemistryexamination.
 32. The system as in claim 31, wherein immunohistochemistryincludes fluorescent labeled antibody staining.
 33. The system as inclaim 28, wherein more than about 40 percent of the tissue samplemaintains its anatomical structure and remains biochemically activeafter thaw.
 34. The system as in claim 28, wherein more than about 80percent of the tissue sample maintains its anatomical structure andremains biochemically active after thaw.
 35. The system as in claim 28,wherein more than about 85 percent of the tissue sample maintains itsanatomical structure and remains undamaged.
 36. The system as in claim28, wherein the cooling fluid is maintained at a temperature of betweenabout −20 degrees centigrade and about −30 degrees centigrade.
 37. Thesystem as in claim 28, wherein the velocity of the cooling fluid pastthe tissue sample is about 35 liters per minute per foot of coolingfluid through an area not greater than about 24 inches wide and 48inches deep.
 38. The system as in claim 28, wherein, the circulator is amotor/impeller assembly immersed in the cooling fluid.
 39. The system asin claim 28, wherein the cooling fluid is circulated past a multi-pathheat exchanging coil submersed in the cooling fluid, and wherein theheat exchanging coil is capable of removing at least the same amount ofheat from the cooling fluid, as the cooling fluid removes from thetissue sample.